This invention relates to optical communication devices and in particular to a fiber optic filter useful in wavelength division multiplexing and demultiplexing. The invention is particularly useful for reducing the wavelength shift of filtering devices against the temperature change. This invention also has applications in altering the wavelength or other optical characteristics of filters and other optical devices.
In recent years, optical fiber technology for telecommunication has progressed rapidly. While the theoretical transmission capacity of the single mode optical fiber has been recognized in the industry as extremely high from the day such fiber was introduced, much of the capacity has not been utilized. For the increasing demand for bandwidth, such as in the transmission of video images and graphics, much attention has been directed lately toward the maximal utilization of the bandwidth of the single mode fiber. Wavelength division multiplexing (WDM) is one of the most viable schemes of maximizing bandwidth utilization of single mode fiber.
In a WDM system, multiple signal sources emitting at different wavelengths, .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n, are coupled into the same single-mode fiber by means of a multiplexer. After the signals of different wavelengths are transmitted through the fiber to a desired destination, the multiple wavelength signals carried by the respective multiple wavelengths must then be separated by means of a demultiplexer into separate optical channels, each wavelength being carried by a different channel to a detector.
WDM based systems have evolved rapidly from early two channel systems to the current 16 channel system. International Telecommunication Union (ITU) has even proposed a 45 channel system utilizing wavelength range from 1533 to 1565 nm with channel spacing of 100 GHz (about 0.8 nm). Furthermore, a WDM system of channel spacing of 50 GHz is being fostered. It challenges optical component manufacturers to provide ultra-narrow bandwidth filters with highly stable pass wavelength against environmental temperature change. This invention is related to the enhancement of the temperature stability of the pass band of the filtering devices.
WDM multiplexers and demultiplexers can be made by employing thin film filters, diffraction grating, waveguides, Bragg in-fiber grating. The WDM employing thin film filters is widely used because of its excellent optical characteristics such as lower loss and higher channel isolation comparing with other technologies.
Filters are usually formed by a stack of thin films made by the deposition processes. The control of center wavelength accuracy of the filter during the deposition process and its stability against temperature change are extremely challenging, particularly for the dense WDM such as 100 GHz spacing system. The shift of the center wavelength under temperature change of 100 degree C. is required to be within 0.1 nm or smaller to prevent the shift from interfering with the adjacent channels.
The optical thickness of thin films responds to the temperature change and therefore the location of the center wavelength shifts with the temperature. The temperature coefficient of the center wavelength shift depends on the film structure, film materials, deposition process and others. Lower temperature coefficient is desired. The typical temperature coefficient of the filter made by the state of art deposition process ranges from 0.003 to 0.01 nm.degree. C. A temperature change of 100.degree. C. would cause a wavelength shift of about 0.3 nm to 1 nm, which is too large for dense WDM applications.
It is therefore desirable to introduce an improved filter system with a stable characteristic frequency over a large temperature range.